Method of making cathodic electrodes for electrolysis furnaces



Dec. 27, 1955 BONNQT 2,728,109

METHOD OF MAKING CATHODIC ELECTRODES FOR ELECTROLYSIS FURNACES FiledSept. 29, 1952 JL H p H /E I? 5 5 INVENTOR.

Maurice Bonnor BY WM "m HIS A TTOR/VE Y5 United States Patent O METHODOF MAKING CATHODICELECTRODES FOR ELECTROLYSIS FURNACES Maurice Bonnot,Petit Coeur, France, assignor to Societe des Electrodes et RefractairesSavoie, a corporation of France Application September 29, 1952, SerialNo. 312,104

Claims priority, application France June 6, 1952 3 Claims. (Cl. 18-54.7)

The present invention relates to cathodic electrodes 7 for electrolysisfurnaces.

It is known that aluminum is generally manufactured by electrolysis ofalumina, dissolved in a; melt of cryolite. This electrolysis is carriedout in special furnaces in which electric current is supplied throughcarbon electrodes, one of which, the upper electrode, plays the part ofan anode, the lower electrode playing the parts both of a cathode and ofa refractory container for the electrolysis melt.

The anode is frequently obtained from a mixture of carbon grains anddust with a hydrocarbon binder, said mixture being shaped by extrusion,compression or any other method and then fired in special furnacesbefore being placed in the electrolysis tank. It is also known, however,to use so-called self firing anodes, consisting of a casing, generallymade of aluminum sheet, filled with an unfired or crude paste of carbongrains and dust mixed with a hydrocarbon binder; such anodes, placed inworking position when crude, are fired in the very furnace in which theyare used. This method leads, among other advantages, to the saving ofthe preliminary firing of the paste forming the anode.

Along the same line it could be conceived to save also the preliminaryfiring of the cathode and to place crude cathodes in the electrolysistank. However, considering that the cathode plays also the part of arefractory container for the electrolysis melt, said cathode must beabsolutely tight and, particularly, must offer no crack which could leadto the penetration of the electrolysis melt therein. Thus a cathode maybe quite pervious to melted metal and, generally, to the electrolysismelt, even when the permeability of the individual blocks which form itis low, if the blocks are poorly bonded together. The penetration of themelt inside the cathode causes, among other drawbacks, a poor quality ofthe metal produced and a'rapid destruction of the cathode which makes itnecessary to interrupt production prematurely.

When setting a tank into operation, the voltage drop in the cathode,which is difiicult to measure with any accuracy, is of the order of 200to 250 millivolts. This voltage drop begins to increase after a fewmonths of operation, it then increases rapidly and 'at the time when itis found necessary to put the tank out of working, the said voltage dropgenerally reaches from 500 to 1000 millivolts, as an average 600 to 700:millivolts. The

difficulty of obtaining tight cathodes caused the unburnt cathodes to beabandoned long ago. Consequently, cathodes are generally made out ofpreviously fired carbon blocks, separated from one another by joints ofsmall dimensions filled with a crude brasque consisting of a mixture ofcarbon grains and dust with a hydrocarbon binder generally having a lowmelting point. It is therefore always necessary to perform a previousfiring 0f the individual blocks.

It is also known to manufacture cathodes by tamping crude brasque overtheir whole area and thickness, the firing being carried out in theproper furnace in which they are to be used. Such a method, however,involves some difficulties and drawbacks, the main four of which aredetailed hereafter.

Generally, no powerful hydraulic equipment is available on the spot forshaping the brasque or paste; for this operation, low power pneumatic orelectric tampers have to be used, which are unable to compress the pasteon a large thickness as must be the case for a cathode (400 to 500 mm.).Therefore, the paste must be tamped in successive applications, i. e. infairly thin layers not exceeding 5 cm. in thickness, which are tampedone after the other. This method of shaping causes a tendency tolaminating, with the consequence of a separating of the various layersfrom one another, and of a penetration of the bath inside the cathode,leading to its premature destruction.

During the tamping operation, the paste delivered from the mixers isactually at a temperature much higher than that of the melting point ofthe binder; but it has to be spread in thin layers, over a large area(that of the tank), in a medium which is at room temperature (much belowthe melting point of the binder). Such operative conditions arefavourable to a rapid cooling of the paste. Now, the viscosity ofhydrocarbon binders usually employed in the electrode industry doubleswhen the temperature drops by 7 degrees centigrade. The result is thatthe viscosity of the paste during the tamping varies from one moment tothe next and may become such that the shaping of said paste bepractically impossible. Particularly, pastes usually employed forelectrodes, and in which the binders are dry pitches having high meltingpoints (70 C. and above) cannot be used in such conditions; pastes mustbe used which contain binders with low melting points (4045 C.). Suchbinders however have the drawback of having a lower, fixed carboncontent (coking residue), and, consequently less favourableagglomerating properties.

It is difiicult to tamp the paste inside a mould having large horizontaldimensions. Now the aluminum tank, before it is lined, actually formssuch a mould. Under the action of the tamping tool, the paste, which isnot held by adjacent vertical walls, is pushed horizontally around thetamping tool, which is also a favourable con dition for laminating. Bypartitioning the tank and tamping the fractions thus constituted oneafter the other, this phenomenon is only restricted.

Any shaping process such as extrusion, compression, vibration, tamping,ramming, and the like, creates an orientation of the paste layers orstrands, and of the particles which form said paste; the result is thatproducts intended to lead electric current, and manufactured by means ofsuch processes have, a resistance to the flow of current, which is aminimum in the plane of the layers or in the direction of the strandsand a maximum in the perpendicular direction. Now it is well known thatthe method of manufacturing of cathodes by tamping of brasque leads to ahorizontal orientation of the paste layers, and of the particles whichform said paste. A cathode manufactured by such a method, thereforeopposes a maximum resistance to thefiow of current which circulatesvertically between the anode and cathode, through the bath, which amongother drawbacks, increases the voltage drop in the cathode.

It was already known, further, in other industries, to use initiallycrude refractories. Thus the hearth, base block and even boshes in blastfurnaces are sometimes made of a tamped, crude carbon brasque or mass.In open-hearth furnaces also, crude magnesia bricks and, in some otherfurnaces, crude prefabricated blocks of refractory chamotte and meltedcement have been used.

It has heretofore been impossible, due to the above mentioneddifliculties and drawbacks, to prepare crude carbon cathodes foreiectrolysis furnaces, such as those used for aluminum, presentinglasting qualities and, at the same time good working conditions.

Now, it has been found that, when starting from crude carbon blocks,excellent cathodes may be obtained, which obviate the drawbacks metheretofore.

The improved method according to the present invention comprises formingcrude blocks of carbon by shaping, for instance moulding or extrudingunder pressure, a crude paste consisting of carbon and a hydrocarbonbinder while keeping said paste at a uniform degree of viscosity,assembling said blocks to the shape desired for the cathode, bindingthem together by means of crude brasque joints, and firing the soobtained cathode in the electrolysis furnace proper.

For the preparation of individual blocks, all usual hydrocarbon bindersmay be used, including those having a high melting point, a high fixedcarbon content andconsequently a maximum of agglomerating properties.

The carbon which is used is not different frorngthat generally used forthe manufacturing of electrodes.

The blocks are shaped in temperature controlled capacities (moulds,dies, or the like), which are adapted to keep the paste, during theentire shaping operation, at a temperature and consequently at aviscosity which are uniform throughout the mass. Furthermore, theshaping may take place under a very high pressure, which may also becontrolled.

The blocks or bars thus obtained may be cut and, gen erally speaking,machined in such a manner that they can always be arranged with thedesired orientation of the layers or strands of paste and of theparticles which form said paste, and particularly with' such anorientation that the said layers and particles are parallel with thedirection of the flow of current through the electrolysis tank, in orderthat the cathode offers a minimum resistance to the flow of suchcurrent.

By operating according to the present invention, a series of advantagesare obtained, either from the point of view of production or from thestandpoint of the properties imparted to the manufactured electrodes.The cost of unburnt blocks is generally lower than that of pre-firedblocks and the positioning of a cathode according to the inventionrequires less labour than that of a cathode made entirely of brasque.

In addition, in view of the fact that the physical condition of thecarbon blocks and the brasque paste forming the joints is the same, anexcellent bonding is thus obtained between the blocks and consequentlythe cathode is truly a one piece member. The carbon blocks themselves,made in one piece, for instance by extrusion in powerful hydraulicpresses, do not have the above men-v tioned lack of homogeneity and, inparticular, are not laminated. These combined factors impart to thecathodes according to the invention a substantially longer life thanthat of known cathodes.

Furthermore, all the above recalled diificulties and complicationsresulting from a premature cooling of the paste under working, areavoided and it is possible to use, r l cratina, 'dt i ches with h h meln Po nts, a as he hi hest sslomerat u p r- F al y, a the time'whea hat ia se is. s arted; t e blocks of crude carbon, passing through a plasticcondi- 4 tion, are in a better condition for withstanding deforma-=tions due to possible outside stresses (expansion stresses forinstance), than prefired blocks which have been already subjected toinner stresses resulting from their firing. 1

Two embodiments of cathodes manufactured according to the invention aredescribed hereafter, reference being had to the appended drawings, inwhich:

Fig. 1 is a perspective view of a plane cathode formed of juxtaposedbars;

Fig. 2 is a view similar to Figure l of a modified construction madeotrt of smaller elements.

The cathode shown in Figure 1 is formed of bars 1 having a squaresection and arranged side by side. Said bars comprise grooves Z forhousing current input rods 3. They are united together longitudinally bybrasque j nts The cathode shown in Figure 2 consists of substantiallycubical bloclgs 19 arranged checkerboard fashion and connectedtogetherby brasque joints 11.

In both cases, the cathode assembly is in a crude condition. Verticalarrows C show the direction of the current flow.

The first type of embodiment which corresponds to Figure 1 calls forsquare section bars 1, for instance 500 by 500 mm. having a length of2,408 meters.

Said bars were obtained as follows:

A mixture containing 82% of strongly calcined anthracite grains anddust, and 18% of coal tar pitch having a iKramer melting point of 85 C.was kneaded to a paste, Said paste was placedat a temperature of C. in a6.000 metric tons extrusion press and extruded thereby through a diehaving a square opening of the size required for the bar. The extrudedbars were cut to the desired length of 2.400 meters and machined byplaning to form the grooves 2 for the rods 3. After having been fittedwith said rods, the bars were arranged horizontally on the bottom of thetank, parallel to one another the direction of their extrusion F ishorizontal), said bars being separated laterally from one another byjoints 25 mm. wide. Then a brasque was kneaded, consisting of 84% ofstrongly calcined anthracite fine grains and dust and 16% of a coal tarpitch having a Kramef melting point of 45 C.; this brasque was thenintroduced into the joints at a temperature of 63 to 70 C., by anexhaustive tamping by means of pneumatic tampcrs. A one piece, crudecathode was thus obtained, offering little resistance to the currentflow.

The Kramer melting point may be defined as being the temperature atwhich a drop of mercury, placed on top of a plug of solid pitch,arranged at the bottom of a tube dipping into a liquid which is beingheated, goes through said pitch plug when the latter melts due to theheating of the liquid. This test is delicate and the prescribedoperating method must be followed very strictly to obtain resultscapable of duplication.

As a modification of this first embodiment, bars prepared in the sameconditions as above were cut, on their issuing from the die, into cubes10, with sides of 500 mm. These cubes were machines, as above mentioned,and positioned in horizontal rows provided with a continuous groove 2,each block being oriented so that the direction of extrusion F (Figure2) be vertical. The cubes were separated from one another in alldirections, by joints 2 5 mm. wide which were filled with crude brasqueobtained and introduced according to the above described method.

In anot er mbo im n a pa e ha in t e 8mm comos t as abo c b d wa shapedn a c p e sion press, under a pressure of 600 kg/sq. cm., into cubes 10as 5.00 sides. Afiej' machining said cubes for providin gr o e t ese boc s w arranged on the bottom f; the so ha the di e i compression P whetizoatal. (F g e he Flaws of orientation at the paste then beingvertical, as shown at 12), said blocks being separated from one another,in all directions, by 25 mm. joints which were filled with crude brasqueaccording to the above mentioned method.

What I claim is:

l. A method for the manufacture of a one-piece cathode for anelectrolysis furnace, comprising the steps of preparing unfired carbonblocks by shaping under pressure a paste consisting of comminuted carbonand a hydrocarbon binder While keeping said paste at a uniform degree ofviscosity, assembling in said furnace said bloclzs in the shape desiredfor the cathode and to constitute composition as said blocks and firingthe obtained cathode in said electrolysis furnace.

2. A method according to claim 1, wherein the unfired carbon blocks areprepared by compressing the paste in molds under high pressure at such atemperature that a uniform viscosity is preserved throughout the massduring the whole compressing operation.

3. A method according to claim 1, wherein the unfired carbon blocks areprepared by extruding the paste under References Cited in the tile ofthis patent UNITED STATES PATENTS 538,289 Shrewsbury Apr. 3-3, 18951,556,990 Henry Oct. 13, 1925 1,734,811 Kalb Nov. 5, 1929 1,899,064Storey Feb. 28, 1933 2,252,277 Tate et al. Aug. 12, i941 2,373,142Hurter June 12, 1945 2,403,301 Richon July 2, i946 FOREIGN PATENTS58,956 Germany Oct. 10, 1391 564,167 France Oct. 15, 1923

1. A METHOD FOR THE MANUFACTURE OF A ONE-PIECE CATHODE FOR AN ELECTROLYSIS FURNACE, COMPRISING THE STEPS OF PREPARING UNFIRED CARBON BLOCKS BY SHAPING UNDER PRESSURE A PASTE CONSISTING OF COMMINUTED CARBON AND A HYDROCARBON BINDER, WHILE KEEPING SAID PASTE AT A UNIFORM DEGREE OF VISOCITY, ASSEMBLING IN SAID FURNACE SAID BLOCKS IN THE SHAPE DESIRED FOR THE CATHODE AND TO CONSITUTE SUBSTANTIALLY THE WHOLE AREA DESIRED FOR THE CATHODE EXCEPT FOR RELATIVELY NARROW JOINTS BETWEEN THE BLOCKS, 